It has been found that, with compression ignition internal combustion engines, the levels of noise and particulate emissions produced by the engine can be reduced by varying the rate at which fuel is supplied during each fuel injection cycle. For example, an injection cycle may include an initial phase during which fuel is supplied at a relatively low rate and a subsequent phase during which fuel is delivered at a higher rate. Alternatively, or additionally, other fuel injection characteristics may be varied. It is an object of the invention to provide a fuel injector meeting these requirements.
According to the present invention there is provided a fuel injector comprising a nozzle body provided with a first bore and defining a first seating, a valve member engageable with the first seating to control fuel flow from the first bore towards a first outlet opening located downstream of the first seating, the valve member being provided with a second bore defining a second seating, a valve needle being slidable within the second bore and being engageable with the second seating to control fuel flow from the second bore towards a second outlet opening provided in the valve member, and a transmission arrangement whereby movement of the valve needle beyond a predetermined position is transmitted to the valve member.
The nozzle body may be provided with a plurality of first outlet openings and the valve member may be provided with a plurality of second outlet openings.
The first and second outlet openings may be located such that, when the valve member engages the first seating and the valve needle is lifted from the second seating, the first and second outlet openings are arranged in series with one another, movement of the valve member away from the first seating permitting fuel delivery through the first outlet opening, by-passing the second outlet opening. It will be appreciated, that depending upon the shape and size of the first and second outlet openings, the fuel delivery rate or other fuel injection characteristics may be varied by varying the distance by which the valve needle is moved.
The number of first outlet openings may be equal to the number of second outlet openings. Alternatively, fewer second outlet openings may be provided, in which case only some of the first outlet openings are used to deliver fuel whilst the valve member engages the first seating and the valve needle is lifted away from the second seating.
Alternatively the valve member may include a tip portion provided with an opening to allow passage of fuel from the second bore to the first outlet opening, the tip portion being arranged to partially cover the first outlet opening when the valve member engages the first seating and being moveable to a position in which the first outlet opening is not covered by the tip portion.
The second outlet opening in the valve member may be an axially extending opening. Movement of the valve needle away from the second seating by an amount which does not exceed a predetermined amount permits fuel to flow from the second bore in the valve member through an axially extending opening into the first outlet opening. As the first outlet opening is partially obscured by the tip portion, the rate at which fuel is delivered is limited. As movement of the valve needle is not transmitted to the valve member, fuel does not flow past the first seating directly to the first outlet opening. Thus, fuel delivery occurs at a relatively low fuel delivery rate. Further movement of the valve needle away from the second seating beyond the predetermined amount results in movement of the valve member away from the first seating and thus permits fuel to flow from the first bore, past the first seating, directly to the first outlet opening, therefore by-passing the axially extending opening in the valve member. The movement of the valve member results in the tip portion moving to a position in which it does not throttle fuel delivery, thereby permitting a higher fuel delivery rate.
The tip portion may be of truncated, conical form, the tip portion partially covering the first outlet opening in the nozzle body when the valve member engages the first seating such that fuel flow through the opening is throttled, thereby permitting a low fuel delivery rate.
This embodiment of the invention provides the advantage that accurate radial guidance of the valve needle is not essential. The construction of the fuel injector is therefore less complex and manufacture is simplified. Additionally, the first outlet opening can be located at a lower axial position in the nozzle body, as there are no openings in the valve needle with which the first outlet opening must align. Thus, the allowable pressure limit for the injector can be increased.
The fuel injector conveniently includes a sac formed downstream of the first seating and communicating with the first outlet opening formed in the nozzle body, the axially extending opening in the valve member communicating with the sac to enable fuel to flow through the axially extending opening into the sac and out through the first outlet opening in the nozzle body. Preferably, the sac is of truncated conical form so as to minimise the dead volume.